The structure, physical properties, purity, and chemical inertness of pyrolytic boron nitride (PBN) make it an attractive container material for elemental purification, compounding, and growth of semi-conductor crystals. Examples include containers for liquid-encapsulated Czochralski (LEC) and vertical gradient freeze (VGF) growth of GaAs and other III-V and II-VI compound single crystals, and source containers for deposition of metals and dopants at high temperatures and ultra-high vacuum by molecular beam epitaxy (MBE). Molecular beam epitaxy equipment is essentially a vacuum furnace in which semi-conductor substrates are coated with epitaxial layers of a variety of elements or compounds of aluminum, gallium, arsenic, indium and the like, by vaporization of these elements or compounds contained in a pyrolytic boron nitride crucible. During practice of the conventional molecular beam epitaxy process, defects in the epitaxial layer structure can occur. There are a variety of causes of such defects with one cause being due to the condensation on the relatively cool internal wall of the crucible generally adjacent to its open end, which results in droplets falling back into the melt. This can result in oval defect levels that can seriously limit the integrated circuit yield obtainable on molecular beam epitaxy wafers. Oval defects are surface dislocations oriented along the 110 crystalographic direction.
The precise control of the temperature uniformity or profile for external heated crucibles is a problem that can affect the quality of vapor deposited epitaxial layers To correct for this non-uniform temperature profile of external heated crucibles, it has been suggested to apply a coating of pyrolytic graphite onto the external surface of the crucible adjacent the open end. Pyrolytic graphite is an anisotropic material that exhibits a thermal conductivity of 0.700.degree. watt/m.degree. C. in the "ab" plane and 3.5 watt/m.degree. C. perpendicular to the "ab" plane. This proposal provided a solution to alleviate the Problem in which the section of the crucible, generally adjacent its open end, was relatively cooler than the remainder of the crucible. In addition, since the crucible is generally heated by external electrical heating means and since pyrolytic graphite is electrically conductive, there is always a problem that the heating means could contact the pyrolytic graphite coating and cause electrical shorting.
Pyrolytic boron nitride can be produced by various methods such as the method disclosed in U.S. Pat. No. 3,152,006 in which pyrolytic boron nitride is produced by the vapor-phase reaction of ammonia and boron halides, such as boron trichloride. By depositing the boron nitride produced in this manner upon a suitable mandrel, such as a graphite mandrel, a wide assortment of shapes can be produced.
It is an object of the present invention to provide an improved crucible suitable for external heating and having a more uniform or controlled temperature profile.
It is another object of the present invention to provide an improved crucible suitable for use in molecular beam epitaxy.
It is another object of the present invention to provide a process for producing an improved crucible suitable for being externally heated and having a more uniform or controlled temperature profile.
The foregoing and additional objects will become fully apparent from the following description and the accompanying drawings.